1. Field of the Invention
The present invention relates to a high frequency module used in a communications apparatus for a wireless local area network (LAN), for example.
2. Description of the Related Art
Attention has been recently drawn to a wireless LAN that forms a LAN through the use of radio waves as a technique for constructing a network easily. A plurality of standards are provided for the wireless LAN, such as the IEEE 802.11b that uses a 2.4 GHz band as a frequency band and the IEEE 802.11a and the IEEE 802.11g that use a 5 GHz band as a frequency band. It is therefore required that communications apparatuses used for the wireless LAN conform to a plurality of standards.
In the communications apparatus for the wireless LAN, a circuit portion (hereinafter called a high frequency circuit section) that is connected to antennas and processes high frequency signals is incorporated in a card-shaped adapter, for example. In addition, it is expected that the communications apparatus for the wireless LAN is installed in a mobile communications device such as a cellular phone. A reduction in size of the high frequency circuit section is therefore desired.
A type of mobile communications device such as a cellular phone is known, wherein a high frequency circuit section is formed as a module operable in a plurality of frequency bands. For example, Japanese Published Patent Application (hereinafter referred to as JP-A) 2003-152588 discloses a module incorporating two diplexers and a single switch circuit. In this module the switch circuit switches one of the two diplexers to be connected to the single antenna. Each of the diplexers separates two signals in different frequency bands from each other.
Each of JP-A 2003-298305, JP-A 9-36604 (1997) and JP-A 9-261110 (1997) discloses a switch circuit in the form of a module.
Each of JP-A 2001-136045 and JP-A 2001-119209 discloses a module incorporating a duplexer for separating transmission signals from reception signals. This module incorporates two band-pass filters each of which is made up of inductor conductors and capacitor conductors of a layered structure made up of a plurality of insulating layers, a plurality of inductor conductors and a plurality of capacitor conductors. Furthermore, these two publications disclose a technique in which the axis of the inductor conductors making up one of the band-pass filters is made orthogonal to the axis of the inductor conductors making up the other of the band-pass filters. In addition, the two publications disclose a diplexer as another example of the module to which the above-mentioned technique is applied. These publications disclose that the diplexer is made up of a combination of a low-pass filter and a high-pass filter, for example.
In the module disclosed in each of JP-A 2003-152588, JP-A 2001-136045 and JP-A 2001-119209, a transmission circuit for processing transmission signals and a reception circuit for processing reception signals are located inside the layered structure. As a result, electromagnetic coupling may be established between the transmission circuit and the reception circuit in such a module. If such coupling is established, there arises a problem that transmission signals leak from the transmission circuit to the reception circuit and reception signals leak from the reception circuit to the transmission circuit and that the isolation between the transmission circuit and the reception circuit is thereby reduced.
This problem becomes noticeable as the module is reduced in size. Therefore, this problem interferes with a reduction in size of the module.
JP-A 2003-298305 discloses a technique in which ground portions of two switches are located between the two switches. JP-A 9-36604 and JP-A 9-261110 disclose a technique in which two systems of switch circuits are symmetrically disposed. However, it is impossible through these techniques to prevent electromagnetic coupling between the transmission circuit and the reception circuit connected to the switch circuits and to thereby improve the isolation.